1. Field of the Invention
The present invention relates to a compressor and, more particularly, to an apparatus and method for controlling an operation of a reciprocating compressor.
2. Description of the Related Art
In general, a reciprocating compressor is operated to suck, compress and discharge a refrigerant gas by reciprocally and linearly moving a piston in a cylinder provided therein.
The reciprocating compressor is divided into a compressor using a recipro method and a compressor using a linear method according to how the piston is driven.
In the compressor using the recipro method, a crank shaft is coupled with a rotary motor and the piston is coupled with the crank shaft in order to change a rotating force of the rotary motor to a reciprocating motion.
In the compressor using the linear method, the piston connected with an actuator of a linear motor is linearly moved.
The reciprocating compressor using the linear method does not have such a crank shaft for changing the rotational motion into the linear motion, causing no frictional loss by the crank shaft, so it has high compression efficiency compared with a general compressor.
The reciprocating compressor can be employed for a refrigerator or an air-conditioner to control cooling capacity of the refrigerator or the air-conditioner by varying a compression ratio of the reciprocating compressor which can be varied by changing voltage inputted the motor of the reciprocating compressor.
Thus, when the reciprocating compressor is employed for the refrigerator or the air-conditioner, the cooling capacity can be controlled by varying the compression ratio of the reciprocating compressor by varying a stroke voltage inputted to the reciprocating compressor. Herein, the stroke refers to a distance between a top dead center and a bottom dead center of the piston.
The reciprocating compressor according to the related art will now be described with reference to FIG. 1.
FIG. 1 is a schematic block diagram showing the construction of an apparatus for controlling an operation of the reciprocating compressor.
As shown in FIG. 1, the related art apparatus for controlling an operation of the reciprocating compressor includes: a current detection unit 4 for detecting current applied to a motor (not shown) of a reciprocating compressor 6; a voltage detection unit 3 for detecting voltage applied to the motor; a stroke calculation unit 5 for calculating a stroke estimate value of the reciprocating compressor 6 based on the detected current and voltage values and a parameter of the motor; a comparing unit 1 for comparing the calculated stroke estimate value with a pre-set stroke reference value and outputting a difference value according to the comparison result; and a stroke control unit 2 for controlling an operation (stroke) of the compressor 6 by varying the voltage applied to the motor by controlling a turn-on period of a triac (not shown) connected in series to the motor according to the difference value.
The operation of the apparatus for controlling an operation of the reciprocating compressor will now be described with reference to FIG. 1.
First, the current detect unit 4 detects current applied to the motor (not shown) of the compressor 6 and outputs the detected current value to the stroke calculation unit 5.
At this time, the voltage detection unit 3 detects voltage applied to the motor and outputs the detected voltage value to the stroke calculation unit 5.
The stroke calculation unit 5 calculates a stroke estimate value (X) of the compressor by substituting the detected current and voltages values and the parameter of the motor to equation (1) shown below and applies the calculated stroke estimate value (X) to the comparing unit 1:
                    X        =                              1            α                    ⁢                      ∫                                          (                                                      V                    M                                    -                  Ri                  -                                      L                    ⁢                                          i                      _                                                                      )                            ⁢                              ⅆ                t                                                                        (        1        )            wherein ‘R’ is a motor resistance value, ‘L’ is a motor inductance value, α is a motor constant, VM is a voltage value applied to the motor, ‘i’ is a current value applied to the motor, and ī is a time change rate of the current applied to the motor. Namely, ī is a differential value (di/dt) of ‘i’.
Thereafter, the comparing unit 1 compares the stroke estimate value and the stroke reference value and applies a difference value according to the comparison result to the stroke control unit 2.
The stroke control unit 2 controls stroke of the compressor 6 by varying the voltage applied to the motor of the compressor 6 based on the difference value.
This operation will now be described with reference to FIG. 2.
FIG. 2 is a flow chart illustrating the processes of a method for controlling an operation of the reciprocating compressor according to the related art.
First, when the stroke estimate value is applied to the comparing unit 1 by the stroke calculation unit 5 (step S1), the comparing unit 1 compares the stroke estimate value and a pre-set stroke reference value (step S2) and outputs a difference value according to the comparison result to the stroke control unit 2.
When the stroke estimate value is smaller than the stroke reference value, the stroke control unit 2 increases the voltage applied to motor to control the stroke of the compressor (step S3), and when the stroke estimate value is greater than the stroke reference value, the stroke control unit 2 reduces the voltage applied to the motor (step S4).
When the voltage applied to the motor is increased or reduced, the triac (not shown) electrically connected with the motor control the turn-on period and applies the voltage to the motor.
The stroke reference value differs depending on a size of a load of the reciprocating compressor. Namely, when the load is large, the stroke reference value is increased not to reduce the stroke of the piston and prevent degradation of cooling capacity. Conversely, when the load is small, the stroke reference value is reduced not to increase the stroke of the piston and prevent an increase of the cooling capacity and generation of collision of the piston and the cylinder due to an excessive stroke (over-stroke).
FIG. 3 is a flow chart illustrating the processes of controlling a top dead center (TDC) of the related art reciprocating compressor. The stroke control unit 2 increases an input current and checks whether a present stroke is TDC=0. If the present stroke is TDC=0, the stroke control unit 2 maintains the present input current as it is.
If the present stroke is not TDC=0, the stroke control unit 2 checks whether the stroke is lower than TDC=0.
If the stroke is lower than TDC=0, the stroke control unit 2 keeps increasing the input current, and if the stroke is higher than TDC=0, the stroke control unit 2 reduces the input current.
Herein, the TDC refers to a position of a piston when a compression stroke of the piston is completed.
A bottom dead center (BDC) is a position of the piston when a suction stroke of the piston is completed.
Efficiency of the reciprocating compressor is maximized at a position where TDC is 0, so when controlling an operation of the reciprocating compressor, the piston is controlled to come to the position where TDC is 0.
In this case, the reciprocating compressor requires particular load conditions, namely, a maximum compression volume.
However, when a gas spring constant is small, the reciprocating compressor cannot be operated with the maximum stroke without collision of the piston because a pushed amount of the piston is small. That is, the reciprocating compressor cannot be operated at the maximum compression volume.